The Rising Tide of Ocean Plagues

How humans are changing the dynamics of disease.

Published: 25-Feb-2006

A leading group of epidemiologists, veterinarians and ecologists report that humans are affecting the oceans in ways that are changing the dynamics of disease. Previously harmless pathogens are becoming killers when combined with contaminants; "good" parasites that invisibly control the balance of species in an ecosystem are disappearing; and changes in sea surface temperature can trigger cholera outbreaks thousands of miles away.

"Human activities are knocking things out of balance," says Andrew Dobson of Princeton University. "For some pathogens, we're seeing nasty synergistic effects with contaminants, such as PCBs. Paradoxically, diseases also play an important role in healthy ecosystem functioning. These changes tend to slip under the radar screen until they show up in ecological cascades that lead to wildlife and human health problems."

Lethal Domoic Acid Poses New Threats to Sea Lions & Humans

Scientists studying sea lions in California are seeing an increase in the number of animals affected by domoic acid, a toxic compound produced by specific types of algal blooms. In high doses, domoic acid is fatal. At lower doses, it can trigger miscarriages and cause gradual, irreversible decay of brain tissue.

Many scientists agree that increases in algal blooms in California and around the world are caused by a combination of factors, including agricultural run-off, oceanographic properties, and global warming. Many of these blooms produce chemicals that are dangerous to humans when concentrated in the food chain. In the case of domoic acid, both shellfish and fish can concentrate the toxin.

Scientists first saw domoic acid poisoning in 1998, when over 70 sea lions died in one weekend. "We thought at the time that it was linked to the fact that it was an El Nino year, but we've seen it every year since then," says Frances Gulland of The Marine Mammal Center in Sausalito, California. "It's a big mystery."

In the last five years, over 1000 animals have died from domoic acid poisoning, and there has been an increase in the number and extent of harmful algal blooms that produce the compound.

"Sea lions eat the same foods we do, but more of them – foods like anchovies, squid, salmon, and mussels – we may see the effects of domoic acid in them before we see it people," says Gulland. Researchers are already seeing signs of serious health problems, such as miscarriages and epilepsy, in sea lions. "Sea lions may be an early warning sign for humans," she says.

While harmful algal blooms are often termed "red tides" because of the color of some algal species, we cannot always see these blooms just by looking at the water. To detect these harmful blooms, scientists monitor plankton and domoic acid levels in mussels. In California, harvesting of mussels and other shellfish is banned when their tissues contain levels of domoic acid over the deadly 200 parts per million. At lower levels, however, there is no public warning system.

"I wouldn't eat shellfish during any sort of harmful algal bloom, especially not if I was pregnant," says Gulland.

Sugar Kills – New Clues in the Mystery of Coral DiseasesIn the struggle to understand and prevent coral diseases, scientists are finding that sugars released in sewage and agricultural run-off play an unexpected role in killing corals.

In the 1970's there were only a few diseases documented on coral reefs. Today there are over thirty, and the numbers are increasing exponentially. "The scary thing is that even in the Great Barrier Reef, one of the most protected reefs in the world, researchers are seeing more diseases every time they look," says David Kline of the Smithsonian Tropical Research Institute.

Nailing down the causes of these diseases has been difficult. Kline and his colleagues, Forest Rohwer and Nancy Knowlton, examine a variety of stressors to understand the origin and spread of coral disease.

Bacterial communities live in healthy corals and are beneficial when kept in check: they may actually protect corals from disease and likely collect and concentrate needed nutrients from the water. But, like an infected cut, rapid growth of bacteria is problematic. It can cause disease or make corals more susceptible to new pathogens.

Sugars that promote this dangerous overgrowth reach the ocean directly from human waste and agricultural run-off, but are also produced by fleshy algae. This link to algae reveals an escalating feedback loop. Nitrogen and phosphorus pollution in run-off enhances the growth of algae that produce simple sugars as they photosynthesize. The sugars, in turn, promote the growth of bacteria, which kills or weakens the coral. This leaves more room for the algae to take over, smothering corals and increasing sugar production, starting the cycle over again and rapidly changing reefs to fields of algae.

In healthy systems, some of this algal growth would be kept in check by fish and other algae-eaters, but overfishing has wiped out many of these populations. "Corals are tough, they've been around for millions of years. But multiple threats such as pollution, overfishing, and global warming may prove to be too much for them," Kline says. "I've seen Staghorn reefs go from brilliant areas that form a nursery for fish, to completely wiped out, in a matter of months."

The good news is that sugar levels can be monitored and significantly reduced. According to recent reports, eighty percent of sewage in the Caribbean is released directly to the water untreated. "I think we can save reefs in the Caribbean and the Pacific," says Kline. "But we need proper water treatment systems throughout the world and large enough networks of marine reserves."

Parasites – Losing Them Can Be Hazardous to Your Health

We typically think of parasites as energy-sucking creatures synonymous with ill-health, but high abundance of parasites in coastal environments can actually be a positive indicator of ecosystem health.

"Parasites are absolutely a bad thing if you're the individual infected by them," says Kevin Lafferty of the US Geological Survey. "But they are a very natural component of ecosystems. Parasitism is the most popular animal lifestyle on the planet."

Because parasites have complex life cycles, often relying on a variety of animals and interactions to survive, they are especially sensitive to environmental change. This susceptibility of parasite populations allows scientists to use the abundance of these small animals as an early detection and monitoring device: a change in the ecosystem will show up as an increase or decrease of parasites before scientists can see those changes in populations of host animals like fish or birds.

Lafferty and his colleagues study small worms called trematodes to monitor the health of salt marshes and wetlands in California. One particular worm starts its life in a snail where it reproduces and is released to the water as a free-swimming larvae that burrows into the tissue of clams, crabs, or fish. Once there, it waits for its new host to be eaten by a bird. In fish, it can hasten the process, swimming to the brain of the fish where it creates a cyst that causes the fish to flash on its side at the surface of the water – making it an easy target for nearby birds. Ultimately, the parasite returns to the water in the feces of birds and starts all over with the next snail encounter.

"Trematodes require all of the pieces of the puzzle to complete their life cycle – when we see a lot of parasites in an estuary, we know it's in good shape. For example, an estuary with high infection rates tells you that it is visited by many birds, and many types of birds," explains Lafferty.

If habitat loss, fishing, hunting, or pollution degrades the system, scientists will see a decline in parasite populations. In the end, reductions in parasite populations may be an equally important indicator of ecosystem malfunction as the more publicly alarming increases in disease.

"Often, we don't realize we miss infectious diseases until we have invasive species that become pests without their natural enemies," explains Lafferty. "Parasites act like thermostats – when a species becomes abundant it is more susceptible to infectious diseases. The disease acts like a safety valve, keeping populations in check."

Climate Variability & Cholera

We are also affecting human health on a global scale by changing the physical factors that influence the spread of disease. In the latest evidence of our ties to the ocean, researchers have discovered how changes in sea surface temperature in the Pacific Ocean are linked to cholera epidemics across the globe in Bangladesh.

To predict and prepare for future outbreaks, cholera expert Mercedes Pascual and her colleagues are studying cholera outbreaks in Bangladesh, where extensive health records stretching back to the late 1800's (when the British Empire was carefully keeping track of disease threats in its colony) and bi-weekly case reports taken during a 1966 surveillance program document disease trends in unique detail. A waterborne diarrheal disease, cholera is still a serious health concern in many areas of the world, including parts of India, South America, Bangladesh, and Africa. It takes a large economic toll on poor communities and, if untreated, can kill in as little as 24 hours.

Pascual and her team have discovered that cholera transmission is highest during high rain and flooding, when sanitary conditions tend to breakdown and people are forced into tight quarters. And now they have reason to believe that these high rain events are linked to warmer ocean conditions in the Pacific during El Nino events. This newly revealed connection to remote sea surface temperatures via increased local rainfall is especially striking because it points to the possibility of using ocean temperatures as an early warning system to predict and prevent disease outbreaks.

"We can think of changes to the oceans locally, but in terms of human health we have to look globally," says Pascual. "This is a global connection. As we change ocean properties through climate change, remote consequences are likely to arise – we may not even know what they are yet."

"As children, if we cut ourselves at the beach, our mothers told us to wash it in the ocean - seawater was always perceived to be clean and healthy. We are seeing ominous signs that this is changing - surfers in California are now advised to have regular hepatitis inoculations and you really don't want to expose an open wound to near shore water," says Dobson. "We should begin to worry deeply about the health of the oceans."

MEDIA NOTE: The scientists will discuss their findings at an AAAS session titled, Rising Tide of Ocean Plagues, on Friday February 17th at 1:45 pm.

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